Effects of Insect Growth Regulators on Ambrosia Beetles (Coleoptera: Curculionidae)

Abstract Ambrosia beetles, especially the granulate ambrosia beetle, Xylosandrus crassiusculus (Motschulsky) (Coleoptera: Curculionidae), are serious pests in ornamental nurseries in Georgia during the spring months. Growers spray pyrethroid insecticides to prevent ambrosia beetle attacks on the tree trunk around bud break. Repeated pyrethroid insecticide applications can harm beneficial arthropods and cause a resurgence of minor, secondary pests. Insect growth regulators (IGRs), such as novaluron and azadirachtin, have demonstrated transovarial activity on many insect pests, for which the viability of the eggs was reduced after adult exposure. IGRs, particularly azadirachtin, are also repellent to many insect pests. Thus, the objective of this study was to determine the transovarial and repellent activity of IGRs on ambrosia beetles. Two experiments were conducted in ornamental nurseries in 2019 and 2021. In the first experiment, novaluron, azadirachtin, and permethrin were sprayed as stand-alone and combination treatments with permethrin on maple (Acer) tree bolts. The number of ambrosia beetle attacks was significantly lower in treatments with permethrin in both years. Novaluron and azadirachtin treatments neither reduced ambrosia beetle attacks on the ethanol-infused bolts nor suppressed the recovery of X. crassiusculus from the bolts, suggesting the lack of repellent and transovarial activity, respectively. For the second experiment, novaluron alone and in combination with 1×, 4×, and 8× bark penetrant at the label rate were sprayed on ethanol-infused bolts. None of the novaluron treatments with or without bark penetrant elicited transovarial activity in X. crassiusculus, as the beetle recovery was similar among treatments.